There are a number of techniques known in the prior art for pasteurizing and processing liquid egg. The most widely used approaches involve the use of conventional plate heat exchangers, steam infusion systems, or combinations of both. However, these techniques have limitations because of the physical nature of liquid egg. Plate heat exchangers are very popular in that they are very efficient and relatively simple to use. However, plate heat exchangers are limited in the amount of heat that they can convey to liquid eggs without causing irreparable harm such as undue coagulation, scorching, caking on the heater plate, and the like. Furthermore, the highest effective temperature range over which heating can be accomplished for liquid egg by plate heat exchangers is limited to between about 150.degree. F. to about 160.degree. F., particularly for long term, continuous processing techniques. As a matter of practical reality, plate heat exchangers can only be used to heat liquid whole egg to pasteurization temperatures of between about 140.degree. F. and about 155.degree. F. for an extended period of time.
Steam infusion systems can be used to obtain much higher temperatures such as, for example, 165.degree. F. However, it is necessary to separate the condensed steam and residual water from the liquid egg during processing. The equipment used for steam pasteurizing liquid eggs is also quite expensive and elaborate.
Electroheating has been used successfully for the heat treatment, and principally the cooking, of foodstuffs. A particularly important technique for electroheating is described by Mr. David Reznik in U.S. Pat. No. 4,739,140, incorporated herein by reference. Mr. Reznik found that when an AC electric current at a frequency exceeding 100 Hz is passed through food, heating of the food can be accomplished without substantial electrolysis or reaction between the food product and the electrodes.
Mr. Reznik's patent does not describe the use of concentric electrodes for electroheating food or the advantages of using concentric electrodes to heat liquid egg. Similarly, Mr. Reznik's '140 patent does not describe the particularly advantageous interplay of electroheating cells as described herein. Mr. Reznik's '140 patent also does not describe pasteurizing egg. Therefore, this patent does not appreciate any of the unique qualities or unique problems associated with the continuous pasteurization of liquid egg.
U.S. Pat. No. 4,695,472 (the "'472 patent") issued Sep. 22, 1987, discusses methods and apparatus for extending the shelf life of fluid food products including eggs. The methods and apparatus described involve the repeated application of high voltage, high current density, discrete electrical pulses to fluid food products. Field strengths used are, at a minimum, 5,000 volts/cm. Voltages as high as, for example, 37,128 volts are disclosed. Current densities of at least about 12 amps/cm.sup.2 are also disclosed as are pulse frequencies of between 0.1 and 100. Preferably, the treatment methods according to the '472 patent involves the application of at least 2 and more preferably at least about 5 high energy pulses to the material being treated. The principal embodiments disclosed in the '472 patent involve the use of direct current.
The techniques described in the '472 patent have several disadvantages. Most notably, electrolysis is caused with resultant electrode loss, contamination of the foodstuffs, and, at least in the case of eggs, detrimental coagulation. To mitigate this problem, the '472 patent requires the use of membranes interspersed between the electrodes and the food product being treated. The '472 patent also fails to appreciate that highly functional egg having outstanding storage stability can be obtained without the need for the complicated pulsed electrical treatment disclosed therein.
The '472 reference also does not appreciate the difficulties attendant the continuous pasteurization of liquid egg. In fact, the '427 patent never describes elevating the temperature of liquid egg, and in particular, liquid whole egg to minimum pasteurization temperatures. Specifically, nothing in the '472 patent discusses the problems attendant ensuring the absence of detrimental coagulation. Even though the '472 patent does describe arcing as a potential problem, the only effect attributed to such arcing is damage to the electrodes. There is no appreciation of the effect of such phenomena on the organoleptic qualities of the egg and the viability of further continuous processing. Furthermore, all of the tests conducted in accordance with the '472 patent of liquid egg involve the use of static, bench testing devices at peak voltages of 34,000 volts or higher and currents ranging from between 7,200 and 14,200 amps. No continuous egg processing was undertaken.
The '472 patent does not describe the use of concentric electrodes or the advantages obtained by their use, nor does it describe the advantageous arrangement of electroheating cells described herein.
U.S. Pat. No. 1,775,579, issued Sep. 9, 1930, describes a milk sterilizing apparatus in the form of concentric electrodes. A tube forms the first electrode of a concentric pair of electrodes with a central rod forming the second. Milk entering the tube is first heated by an immersion heater inserted into one end of the tube to a temperature in the range of about 90.degree. F. to about 100.degree. F. The milk, thus heated, is passed between the tube and the rod electrode through the electric field established between them by an electric supply system at 110 volts. No mention is made of using such a device to pasteurize liquid egg and thus the intricacies of such processing is not appreciated by the inventor thereof. This patent also fails to appreciate the advantages obtained by the arrangement of electrical components described herein.
Geren, U.S. Pat. No. 4,457,221, issued Jul. 3, 1984, shows a three phase sterilization apparatus where three parallel electrodes are arranged in a circular pattern to form three equidistant channels through which materials to be treated are passed. Voltages employed are approximately 100 volts RMS with voltages up to 1000 volts preferred. Current densities are at least 50 milliamperes per square centimeter (ma/cm.sup.2) but in practice are between 500 ma/cm.sup.2 and 1.25 ampere/cm.sup.2. Time of the application of the currents are less than 200 micro-seconds and the frequency is 50 Hz. The device of the '221 is described as an apparatus for sterilizing by killing bacteria and similar organisms in a host which may be either solid or liquid. No mention is made of treating liquid egg nor of the problems encountered in the processing of such material. Furthermore, the unique arrangement of elements of the present invention are neither described nor appreciated.
Bushnell et al., U.S. Pat. No. 5,048,404, issued Sep. 17, 1991, are directed to pulsed high voltage systems for extending the shelf life of pumpable food products which include liquid egg products. The liquid foodstuff is pumped through passages between concentric electrodes and subjected to very high voltage pulses of a very short duration. The liquid foodstuffs are pumped at a rate such that it will be subjected to at least one pulse in the treatment zone and preferably to two pulses. The pulse will have a duration of from 0.01 microseconds to about 10 microseconds at pulsed field strengths in excess of 30 KV/cm, preferably about 35 KV/cm. The overall system shown in FIG. 1 introduces preheated liquid foodstuff at about 104.degree. F. to 122.degree. F. from in line heating unit to the long pathway uniform electric field cell where the liquid foodstuff is heated to about 122.degree. F. to 158.degree. F. The liquid foodstuffs are then cooled to 41.degree. F. to 50.degree. F. and packaged.
Bushnell et al. include in this teaching, the substance of the U.S. Pat. No. 4,695,472 patent which it incorporates by reference, particularly as to the use of a membrane to prevent electrolysis. Although the use of the concentric electrode apparatus upon various pumpable products is described in detail the use of the device with liquid egg is not. In fact, no example of continuous processing of liquid egg, or any coagulable liquid is provided. Since the parameters of the pulsed high voltage system is similar to that of the '472 patent it is believed the problems and difficulties noted above with respect to the '472 patent will also be true of Bushnell et al.
Bushnell et al. neither teach the use of high frequency alternating current to avoid electrolysis nor suggest that high frequency electrical energy could be used to pasteurize liquid egg in a long term continuous process. Bushnell et al. are also silent on whether or not concentric electrodes could work with such high frequency energy in a long term continuous process. Moreover, Bushnell et al. do not appear to teach or suggest electroheating whereby the attainment of pasteurization temperatures is a direct result of the application of certain forms of electrical energy.
Instead, Bushnell et al. suggest the addition of electrical pulse energy to a system otherwise at pasteurization temperatures can provide an effective kill. That is to say, Bushnell et al. do not teach methods of pasteurizing or even heat treating, but rather, producing improved microbial kill. This point is emphasized by the fact that Bushnell et al. do not appear to teach or disclose the use of otherwise necessary pasteurization equipment such as holding tubes.
As anyone of ordinary skill in the art can attest, the use of different types of electrical energy in different types of electrical processes brings with it its own set of advantages, disadvantages, and complications. Because, for example, Bushnell et al. do not use high frequency alternating current as is practiced in accordance with the present invention, they have no appreciation of the problems and advantages associated with its use. For example, current densities of at least about 12 amps/cm.sup.2 are disclosed in the '472 patent which is incorporated by reference in Bushnell et al. While such current densities may be possible, in the context of a pulse system when applied to liquid egg, the present inventor believes that such current densities are, even under the most academic conditions, unattainable in the present system. Bushnell et al. fail to appreciate that in a long term electroheating application utilizing continuous application of high frequency AC electric current, too large a current density coagulates egg which can cause it to begin to stick to the surfaces of the electrodes. Caking, arcing and fouling will result shortly thereafter.
The present inventor has learned that, if the electrodes can be cooled efficiently enough, the coagulated egg may not stick to the surface of the electrodes such that one method of controlling, for example, arcing, is the use of very efficient cooling. On the other hand, the application of AC energy with too high a current density can still cause significant and even detrimental coagulation to occur. Of course, by the application of relatively high current densities, and with efficient cooling, it may be possible to coagulate or cook liquid egg. However, if no detrimental coagulation is desirable, as is usually the case for liquid egg, lower current densities must be used even with efficient cooling. The '472 patent and Bushnell et al. exhibit their lack of appreciation of these considerations by failing to even discuss the need for cooling, let alone efficient cooling, or the problems of coagulations which can be brought about by the use of very high current densities during the application of high frequency alternating electric current.
In copending U.S. patent application Ser. No. 07/862,198, filed Apr. 2, 1992, now U.S. Pat. No. 5,290,583 entitled "METHODS AND APPARATUS FOR ELECTROHEATING LIQUID EGG" by David Reznik and Aloysius Knipper, there is described a methodology for the pasteurizing of liquid egg at temperatures which would not otherwise be conveniently available to eliminate pathogenic bacteria without sacrificing the physical characteristics of liquid egg. The electroheating is achieved at a current frequency which is effective to heat the egg without electrolysis or detrimental coagulation. The electroheated liquid egg is held for a period sufficient to achieve pasteurization and then cooled for packaging or storing.
The electroheating cell of the above identified application contains at least one pair of electrodes having egg engaging surfaces spaced apart from one another to form a gap through which the liquid egg to be pasteurized is passed. The electrodes are generally parallel plate electrodes having highly smooth, non-wetting surfaces and having rounded edges which do not contact the liquid egg being processed through the cell. The surfaces of the electrodes are extremely smooth and difficult to wet or adhere to and thus provide a smooth, unimpeded flow of liquid egg through the cell.